The present invention relates generally to ophthalmic lenses and, more particularly, to a multifocal ophthalmic lens adapted for implantation in an eye, such as an intraocular lens, or to be disposed in a cornea, such as a corneal inlay.
The general construction of a multifocal ophthalmic lens is known in the art. U.S. Pat. No. 5,225,858, which is incorporated herein by reference, discloses a multifocal ophthalmic lens including a central zone circumscribed by multiple concentric, annular zones. This patent discloses a means of providing improved image quality and light intensity for near images. The improved image quality is accomplished by maintaining the near vision correction power of appropriate zones of the lens substantially constant for a major segment of the near vision correction power region of each zone, and by providing a central zone having an increased depth of focus.
The major segment of each near vision correction power region, which has a substantially constant near vision correction power, inherently reduces the depth of focus associated with far vision. The location of near focus is typically immaterial for near vision, because of the ability of the user to easily adjust the working distance of the target object. The patent discloses progressive vision correction powers in the central zone for extending the depth of focus. The increased depth of focus provided by the central zone helps to compensate for the reduction in depth of focus associated with the near vision correction power regions. This feature is particularly applicable to an intraocular lens, since the patient has minimal residual accommodation, i.e., the ability of a normal eye to see objects at different distances.
FIG. 1 shows how the multifocal ophthalmic lens 6 of the prior art focuses parallel incoming light onto the retina 10 of the eye. For the normal lighting condition with a 3 mm pupil diameter, the rays 7 pass through a far focus region of the multifocal ophthalmic lens 6, and are focused onto the retina 10. The rays 8 pass through a near region of the multifocal ophthalmic lens 6, and are focused into a region between the retina 10 and the multifocal ophthalmic lens 6.
The multifocal ophthalmic lens 6 shown in FIG. 1 shows the passage of parallel rays through the multifocal ophthalmic lens 6 in a well-lit environment. In low lighting conditions, the pupil enlarges, and additional annular zones of the multifocal ophthalmic lens 6 become operative to pass light therethrough. These additional annular regions operate to provide additional far (rays 10 in FIG. 1) and near-focus corrective powers to the multifocal ophthalmic lens 6. Presence of the additional intermediate and near rays shift the best image quality for far vision to the location in front of the retina. As a result, in low lighting conditions the best quality image of the multifocal ophthalmic lens 6 appears in a region slightly in front of the retina 7. A user looking through the multifocal ophthalmic lens 6 while driving at night, for example, may notice an undesirable halo affect around a bright source of light. The shift in the best quality image just in front of the retina 7 instead of on the retina 7 increases the halo effect making driving for some people difficult.
A problem has thus existed in the prior art of providing a multifocal ophthalmic lens, which can provide a desirable far vision correction in low lighting conditions, but which does not unnecessarily elevate halos and contrast reductions under increased pupil size which usually occurs in low lighting conditions. Thus, the prior art has been unable to produce a multifocal ophthalmic lens, which achieves a best quality image on the retina (instead of slightly in front of the retina) in low lighting conditions.
Under low lighting conditions, the best quality image of prior art multifocal ophthalmic lenses is not focused on the retina of the eye. Instead, these prior art multifocal ophthalmic lenses have a best quality image in front of the retina in low lighting conditions, which corresponds to a mean power of the multifocal ophthalmic lens being slightly higher than the far vision correction power required for the patient.
As light diminishes and pupil size correspondingly increases, the outer annular zones of a multifocal ophthalmic lens begin to pass light therethrough. These outer annular zones traditionally introduce additional near vision correction power, which effectively shifts the best quality image from on the retina to an area slightly in front of the retina.
The outer annular zones of the present invention have vision correction powers, which are less than the far vision correction power of the patient, to compensate for the increase in the mean power of the multifocal ophthalmic lens. A multifocal ophthalmic lens, having outer annular zones with vision correction powers less than a far vision power of the patient, is disclosed. The additional annular zone or zones come into play when the pupil size increases under dim lighting conditions, to thereby compensate for the additional near vision annular zones introduced by the enlarged pupil size. The net effect of the additional near vision annular zones and the additional annular zones having power less than the far vision correction power is to focus the best quality image onto the retina of the eye, to thereby reduce halo effects and improve contrast.
The multifocal ophthalmic lens of the present invention is adapted to be implanted into an eye or to be disposed in a cornea, and has a baseline diopter power for far vision correction of the patient. The multifocal ophthalmic lens includes a central zone having a mean vision correction power equivalent to or slightly greater than the baseline diopter power depending upon pupil size, and includes a first outer zone located radially outwardly of the central zone.
A second outer zone located radially outwardly of the first outer zone provides vision correction power, that is less than the baseline diopter power. The vision correction power of the second outer zone can be substantially constant. Light generally does not pass through the second outer zone under bright lighting conditions.
A third outer zone of the multifocal ophthalmic lens comes into play in lower lighting conditions, and includes a vision correction power greater than the baseline diopter power. A fourth outer zone circumscribes the third outer zone, and includes a vision correction power, that is less than the baseline diopter power. This fourth outer zone passes light in very low lighting conditions, when the pupil is significantly dilated. The second and fourth outer zones serve to focus light slightly behind the retina of the eye, to thereby compensate for light focused in front of the retina of the eye by the first and third outer zones, under dim lighting conditions.